A basic technique for retorting oil shale which occurs in vast deposits throughout the world is to heat the oil shale in an oxygen-free atmosphere to a temperature of about 900.degree. F. to convert kerogen to liquid and gaseous products. This basic retorting process has been carried out by mining the oil shale, either by underground or open pit mining, and carrying the oil shale to large retorts where it is heated and the kerogen converted to liquids and gases. An alternative approach which has significant economic advantages and much less environmental impact involves retorting the oil shale in situ. The in situ retort is generally a subterranean cavity or chamber filled by an expanded mass or "rubble pile" of fragmented mass of oil shale particles. The cavity and fragmented mass of oil shale particles can be formed by explosive techniques. The in situ retort is ignited at the top and burned downwardly by an oxygen supplying inlet gas introduced at the top of the in situ retort and withdrawn from the bottom. A combustion zone is formed and moves downwardly through the in situ retort as gas moves from the top to the bottom of the in situ retort. The gases from the combustion zone are at a sufficient temperature to heat the oil shale below the combustion zone to the necessary temperature to convert the kerogen to liquids and product gases. The retorted oil shale contains sufficient carbonaceous materials to sustain combustion when contacted by oxygen.
As discussed in more detail in copending application Ser. No. 536,371, filed Dec. 26, 1974, now abandoned entitled "Method for Assuring Uniform Combustion in In Situ Oil Shale Retorts," by Chang Yul Cha and assigned to the same assignee as the present invention, maximum recovery efficiency is achieved when the combustion zone moves downwardly through the retort as an approximately planar and preferably horizontal combustion zone. Ignition of the rubble pile of fragmented shale has been obtained by burning a combustible gas with air or other oxygen supplying gas and impinging the flame downwardly at the top of the rubble pile through a conduit which admits gas into the top of the cavity. Even where there is more than one ignition area at the top, ignition may not be uniform and burning takes place unevenly throughout a substantial portion of the in situ oil shale retort. While the combustion zone tends to become more planar as the combustion zone advances downwardly, pockets of unburned or unheated oil shale may remain, reducing the overall efficiency of the retorting operation.